The present invention relates to an apparatus and process for recovering silver from chemical solutions having silver dissolved therein.
Many chemical processes, most particularly those related to photographic processing, result in waste fluids containing significant amounts of dissolved silver. Because silver is a semiprecious metal, the value of the silver dissolved in such spent photographic processing chemicals is sufficiently high to support efforts to recover the silver prior to disposing of the waste fluid. Furthermore, silver is a regulated substance with limits set as to how much silver may be present in fluids which are discharged into sewage systems. Thus, there is not only an economic incentive to recover the dissolved silver from such spent processing chemicals, but an environmental imperative as well.
A common method for recovering silver from solution has been developed which employs a process of metal replacement wherein a less expensive metal which is higher than silver in the electromotive series, for example iron, replaces the silver in the solution through a chemical reduction reaction. Prior art devices employing this process generally comprise a vessel which houses the replacement metal or medium. The vessel will generally have at least one inlet port for receiving untreated solution, and at least one outlet port for discharging the treated solution from which the dissolved silver has been removed. The replacement medium is arranged within the vessel in a manner that allows the incoming untreated waste solution to pass over the replacement media, allowing the metal replacement to take place. The recovered silver forms a black sludge-like precipitate that tends to collect in the bottom of the vessel, or adhere to the surfaces of the remaining replacement medium. This "black sludge" tends to clog the system, and as it covers more and more surface area of the remaining replacement medium, lowers the efficiency of the silver recovery apparatus.
Channeling is another problem common to prior art silver recovery units. Channeling occurs as the waste fluid being treated consumes the replacement medium. As the replacement medium is consumed by the waste fluid, small voids or channels develop within the medium where the replacement metal has been depleted. Because the effluent flowing through the silver recovery unit will follow the path of least resistance, these channels create preferential flow paths through the silver recovery core allowing the waste fluid to bypass the silver replacement medium. As a result, less silver is removed from the waste solution, and the efficiency of the silver recovery apparatus is reduced. If the number and size of such channels becomes significant, the silver recovery cartridge will be rendered useless.
The prior art discloses numerous examples of silver recovery units. For example U.S. Pat. Nos. 3,630,505 and 3,692,291 to MacKay disclose silver recovery units comprising a vessel housing a porous metal core formed of a coiled metal screen. Many other prior art references such as U.S. Pat. No. 4,740,244 to Williams and U.S. Pat. No. 5,472,176 to Azzara, to name but a few, disclose silver recovery units employing replacement media in the form of steel wool, iron filings, turnings, chips or powder. Each of the media disclosed in this latter group may be characterized as having large surface-to-volume ratios to increase contact between the spent waste fluid and the replacement medium. U.S. Pat. No. 5,458,024 to Schiller et al. discloses a steel wool silver replacement medium held in place between a pair of non-metal porous pads. Finally, U.S. Pat. No. 5,298,170 to Woog, discloses an effluent neutralization process and chamber wherein silver is removed from spent photographic fixer. This unit includes iron rods disposed within an inlet conduit, which directs incoming fluid down into a mass of steel wool located at the bottom of the chamber. Both the iron rods and the steel wool act as sources of iron to replace the dissolved silver ions. Woog includes a mesh bag containing even more steel wool suspended within the chamber to supply an additional source of iron.
To be viable, a silver recovery unit must efficiently remove silver from the spent waste fluid, reducing the concentration of silver remaining in the treated solution to levels below the minimum desired threshold. Silver discharge is generally regulated at a municipal level and in those jurisdictions where silver discharge is regulated, the allowed concentration of silver discharge is generally limited to less than 5 parts per million (ppm). Further, a silver recovery apparatus must have a reasonably long operating lifetime. Thus, an effective silver recovery unit must remain free from clogging and must effectively remove silver to concentrations below 5 ppm for an extended period of time. The prior art designs generally fail in at least one of these important criteria.
For example, silver recovery units employing only a wound metal screen as the replacement medium are effective at removing large concentrations of silver from solution to bring the silver concentration down to about 200 ppm or higher. However, metal screen media are generally ineffective at "polishing" the fluid to concentrations less than approximately 200 ppm. In contrast, those units employing a more finely divided replacement medium such as steel wool, metal filings, or the like, are effective at removing silver to very low concentrations below about 5 ppm, however, the silver precipitate tends to become enmeshed in the steel wool or other fine media, and tends to clog the device. Channeling is also more prevalent in silver recovery units employing a more finely divided replacement medium.
Another problem which is specific to silver recovery units employed in removing silver from photographic processing waste chemicals is the removal of gelatin from the waste solution. Undeveloped photographic film contains a layer of gelatin which is partially removed during the photographic fixing process. The removed gelatin ends up in the spent photographic fixer solution along with the dissolved silver. When the spent fixer is passed through a silver recovery unit the gelatin tends to deposit or become lodged within the replacement core. Excessive build up of gelatin within the core can interfere with the flow of effluent through the silver recovery unit. Furthermore, as the gelatin becomes embedded in the replacement medium, those portions of the replacement medium covered by the gelatin are effectively blocked from contacting the waste effluent passing through the silver recovery cartridge. Thus, over time a significant amount of the replacement core's surface area is not available for the silver recovery process, lowering the efficiency of the silver recovery unit and shortening the operating life of the device. Therefore an efficient silver recovery unit must provide for the loss of replacement media surface area due to the presence of gelatin, by either providing additional replacement media to compensate for that lost to the gelatin, or removing the gelatin from the solution prior to passing the solution over the replacement media.
In light of the preceding background, there is a need for an improved metal replacement silver recovery unit. It is desirable that such an improved silver recovery unit includes provisions to prevent clogging and channeling within the silver replacement core. It is further desirable that an improved silver recovery unit is designed to increase the longevity of the silver replacement core and efficiently replace silver from waste solutions.
Additional considerations for an effective silver recovery unit include shipping durability and leak resistance. In general, silver recovery units are easily subject to damage during shipping which can cause cracks in the outer vessel, or leakage in and around in the inlet or outlet ports. Thus, an improved silver recovery unit must be designed to include a strong outer vessel to survive unscathed a rough and sometimes hazardous shipping and handling process.